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CNC is an acronym that stands for Computer Numerical Control. It is a term that is used in reference to either the automated specialized machinery (sometimes called machining centers) that can remove material from a stock piece (also called a workpiece or blank) or to the service that performs those machining operations.

CNC machining is a subtractive manufacturing process in that it involves using cutting tools to remove material as needed to form the desired part. This approach contrasts with an additive manufacturing technology, also called 3D printing, in which material is added layer-by-layer to build up a part to its desired final form as opposed to removing material from a blank to generate it.

Before there was any type of automated control of machining, the creation of a custom machined part was performed manually on a lathe or milling machine by a machinist. The work required operators with a high skill level who could read blueprints, take precision measurements using instruments such as calipers and micrometers, and repeatedly cut away at the workpiece until a finished part with the correct dimensions was created. The higher the tolerances needed and the more complex the design, the more tedious the machining task became, which limited throughput and impacted costs. Steps to add automation to the process began to take on increased importance for projects involving the production of highly precise components.

Computer Numerical Control (CNC)
CNC evolved from earlier systems of numerical control that can trace their origins back to the 19th century, where, for example, punched paper was used to control player pianos. Early numerical control programs that were developed in the middle of the 20th century relied in the use of punch tape cards to store instructional code. This code was given the name G-code, the name reflecting the company responsible for its development – Gerber Scientific Instruments

As electronics and semiconductor technology evolved and advanced, more powerful computers in smaller footprints were developed, helping to permit the creation of more sophisticated digital programming. Building off the numerical control foundation, CNC took form and became a more powerful, adaptive, and sophisticated control mechanism that can offer highly repeatable process steps and monitor the quality of the machined workpiece to assure that it meets the dimensional tolerances of the programmed part design.

The CNC Machining Process
The CNC machining process can be divided into four basic steps:

Designing and creating a CAD model representation of the desired part
Doing a conversion to take the CAD model information and generate a CNC program
Performing the initialization or set-up of the CNC machine
Running the stored program to perform the machining operation
CNC Programming
Programming a CNC machine for milling or lathe operations requires an understanding of how to create G-code and M-code.

G-code represents the operational language of the CNC process. The commands used line-by-line provide instruction on how the tooling in the machine should move in order to be in the correct position to perform a cutting operation. For the programming to function correctly, it is necessary to establish a fixed reference frame of three axes so that a command that specifies tool movement to a specific location will do so against this reference frame or coordinate system.

M-code is the machine control language for CNC. It is used to switch on or off certain machine functions. For example, there are commands to start or stop programs, start spindle rotation in counterclockwise or clockwise directions, or turn on or off coolant. When used in conjunction with G-code, the CNC programming generated an automated sequence of steps that can produce a finished part matching the original CAD model in dimensions and desired tolerance.

Learn more about G-code and M-code in our related articles found below.

CNC Operations
There are several specific operations that fall under the general umbrella of Computer Numerical Control or CNC. The most common of these operations are drilling, milling, and turning. Other types of operations include:

To learn more about the specifics on these operations, see our overview articles on understanding CNC machining and understanding CNC milling.

This article presented a brief discussion of the meaning of the term CNC, provided a look at the evolution of the technology from earlier forms of numerical control, and explained more about the CNC machining process and underlying programming code used to control the machining centers. To learn more about CNC machining and milling, explore our other related articles listed below.


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